The present invention relates to a measuring device for non-destructive measurement of nuclear spin distribution, relaxation time distribution, and the like, of an object, by means of nuclear magnetic resonance, and more particularly to a method and a device for accomplishing high-speed three-dimensional imaging.
In 1946, Bloch and Purcell separately discovered the nuclear magnetic resonance (hereinafter, referred to as NMR) phenomenon that, when a nuclei with a finite number of spin quanta such as protons (.sup.1 H) or the like is put into a magnetic field, a radio frequency wave definitively formed by the intensity of the magnetic field and the nuclei is resonantly absorbed thereby. NMR has come to be almost indispensable in the physical and the chemical fields such as structural analysis of a substance or the like. Further, the energy in the magnetic field of the NMR is remarkably small (about 10.sup.-9) in comparison with the radiation energy of X-rays and has almost no effect on living tissue. Thus, there has been rapid progress in the techniques of acquiring local information in living tissue, mainly .sup.1 H as an image by means of the principle of NMR.
In order to accomplish such imaging, it is necessary to spatially resolve the information relating to nuclear spins. To realize this aim, there have been proposed some methods such as the sensitive point method, the projection-reconstruction method and the like.
Of these, as a method for realizing three-dimensional imaging, the method referred as to "three-dimensional Fourier imaging" is disclosed in the Journal of Magnetic Resonance Vol. 18 (1975) pp. 69-83. This is the method wherein first, second and third gradient magnetic fields, crossing one another at right angles, are sequentially applied and a free induction signal (FID) during the period of applying a third magnetic field is measured. The first and the second gradient magnetic fields are respectively utilized for encoding the information of phase positions of the nuclear spins. This method has the disadvantage in that when defining a picture element as M.sup.3, FID must be measured M.sup.2 times and thus when M is large, the measuring time is made very long.
Chemical shift imaging can be considered as one kind of plural-dimensional imaging. This is the method of realizing the three-dimensional imaging as defining one axis of three dimensions as a chemical shift axis, that is, the axis showing a small shift (chemical shift) of a resonance frequency caused by the difference of chemical coupling at one nuclei. Particularly, it is known that the distribution of several phosphorous compounds in a living tissue can effectively inform an observer of the metabolic state of a living tissue. Thus, it is expected that realizing the device for accomplishing the imaging of .sup.31 P chemical shift is a great contribution towards a biochemical diagnosis of a body. One example of chemical shift imaging is shown in, for example, "A. A. Maudsley et. al. Spatially Resolved High Resolution Spectroscopy by Four-Dimensional NMR J. Maga. Reson. 51, 147-152 (1983)". This method is slightly different because it measures an FID with chemical shift without using the third gradient magnetic field against the above-stated three-dimensional Fourier imaging. Thus, it also has the disadvantage that when defining the number of picture elements as M.sup.3, the M.sup.2 time of FID must be measured similarly as above.